The present invention relates to an electrochemical biosensor which can be used for the determination of a specific component (analyte) in a fluid test sample. Electrochemical biosensors of the type under consideration are disclosed in U.S. Pat. Nos. 5,120,420 and 5,264,103. These devices have an insulating base upon which carbon electrodes are printed and are then covered with a reagent layer comprising a hydrophilic polymer in combination with an oxidoreductase specific for the analyte. These devices typically involve a base and a cover which are separated by a generally U-shaped piece as a spacer element, so that when the base and cover are mated there is created a capillary space containing the electrodes covered by the reagent layer. In an alternative structure, the cover is embossed so as to form the capillary space when it is mated with the base thereby eliminating the need for the spacer element. A hydrophilic polymer, e.g. carboxymethyl cellulose or poly (ethylene oxide) is used to facilitate the drawing of the aqueous test fluid into the capillary space.
In either embodiment, working and counter electrodes are screen printed onto the base so that an electrochemically created current can flow when these electrodes are electrically connected and a potential is created between them. Touching the opening in the end of the sensor to a drop of test fluid such as blood results in the fluid being drawn into the capillary space, so that it covers the reaction layer on the surface of the electrode. An enzymatic reaction between the oxidoreductase and the analyte creates a flow of electrons which are carried to the working electrode by a mediator such as ferricyanide and flow through the working electrode to a meter which measures the magnitude of the current flow. The counter electrode serves dual purposes. First, it provides a fixed potential against which the working electrode is controlled. Second, for a two electrode system, such as that depicted in FIGS. 1 and 2, the counter electrode is used to complete the electrical circuit. In this mode, each electron that is transferred to the working electrode is returned to the test fluid at the counter electrode side of the cell. The device's software is programmed to correlate the magnitude of this flow with the concentration of analyte in the test sample. In order for this current to flow, a complete circuit is formed by covering both electrodes with the conductive test sample fluid and applying a potential therebetween.
A problem which is sometimes associated with this type of sensor occurs when an insufficient amount of blood is applied to the opening, so that the counter and working electrodes are not completely covered with the sample. This results in an incomplete current flowing across the electrodes. Since the amount of analyte detected is directly proportional to the current flowing through the detection meter, failure to completely cover the sensor's electrodes can result in an artificially low reading of the sample's analyte concentration. One technique for dealing with this under filling problem is claimed in U.S. Pat. No. 5,628,890 which involves a mechanism for preventing any response from being detected when the sample volume is too low to provide an accurate reading. This design involves a strip comprising an elongated electrode support defining a sample transfer path for directional flow of the sample from a sample application point. A working electrode is placed in the sample transfer path and a counter or reference electrode down stream from the working electrode in the sample transfer path. Failure of the test sample to totally cover the working electrode will result in no response from the reading mechanism due to the absence of a closed circuit through which current can flow.
In co-pending application Ser. No. 09/731,943 there is disclosed an electrochemical sensor of the type described above in which a small sub-element of the non-working electrode is positioned upstream from the working electrode, so that when there is insufficient flow of electrical current through the detector to constitute a valid test for the concentration of analyte in the fluid test sample, the pre-programmed detector causes the emission of an error signal to alert the user of the device that the test result should be disregarded. This is achievable because there is generated an altered current profile in the event the capillary space of the sensor is underfilled. It has been discovered, however, that the use of the sub-element of the counter electrode, sometimes referred to as the trigger electrode, can result in a delayed response in terms of activating the system which delay adds unnecessary time to the duration of the analyte assay. This may be due to the lower initial current being generated by the partially covered working electrode and the small trigger electrode in underfilled sensors.